Android is not secure enough for some pay TV content, most content is fine, but for some premium sports and first run movies Android devices might not be considered secure enough. If someone thinks Android is secure and that I am being ridiculous... then I am afraid you don't have enough experience of 4k uhd ott android set-top box content security requirements from rights holders. I'm not saying they are right to have such strict requirements but they do exist.
To overcome the problem of not being the most prominent app on the TV the operator could make an arrangement with the TV manufacturer to take over the main user experience of the product. The IPTV box operator services would then have the prominence they want. However! Consumer facing TV brands like Samsung don't like to give up the ownership of the consumer in this way and so making such a deal isn't easy. I have seen it done when the content offering is good enough but I have also seen it go wrong.